It was the sort of incident that often triggers anti-Fidel Castro protests, so police quickly kicked into gear.
The nearest policeman fired his pistol over the heads of running youths and within minutes had back-up from a dozen cars, some unmarked. Several youths were bundled into the cars and taken away. They may be released soon, they may not. In Cuba, you just never know.
Nilvio Labrada, a retired army colonel, was not given the luxury of going to jail. After tossing Marxist textbooks from his home last week and shouting "Down with Fidel Castro," he was taken to a mental hospital as neighbours shouted "thugs" and "bullies" at police. For committing the gravest crime of all in Cuba, criticising Mr Castro, he is unlikely to get home soon.
Colonel Labrada's neighbours said he was in no way unstable, but no one was surprised by his fate. Psychiatric hospitals in Cuba are full of government critics, not to mention people with Aids, whom Mr Castro deems unfit to live among the rest of the population.
Despite a series of economic openings in the last couple of years, allowing a small degree of capitalism at individual level, Mr Castro, 67, has not loosened his political grip.
Much-flaunted municipal "elections" next weekend will do nothing to diminish his control. Only candidates nominated by the island's one permitted party, the Communist Party, will be running.
Mr Castro's police, who rely on informants on every block to quell anti- regime protests, are already preparing for possible demonstrations on 13 July. That is the first anniversary of the day a tug hijacked by would- be emigres was sunk by the Cuban coastguard. Forty-one people, including 20 children, were reported drowned.
Exiled Cubans in Miami are trying to organise the protests via Miami- based radio stations but, faced with the threat of a sojourn in a mental hospital, Cubans on the island may settle for switching off their lights for 15 minutes as a discreet protest while exiles in the US take to the streets.
As last year's exodus of rafters showed, whether for reasons of politics or hunger, more and more Cubans want out. Since President Bill Clinton's change of policy in May, shipping rafters back to Cuba proper instead of to US camps at the Guantanamo military base in the east of the island, queues have built up outside the US interests section in Havana in the hope of getting visas.
The island's suicide rate was recently revealed as the highest per capita in the Western hemisphere, almost double that of the US. Among those shocked by the country's deterioration was the former Costa Rican president and Nobel Peace Prize winner, Oscar Arias. Snubbed by Mr Castro during a visit at the head of a Latin American delegation, Mr Arias described the island as "a Latin American Albania ... a gulag ... everyone feels a prisoner.''
Mr Arias then wrote to Mr Castro, saying: "I am alarmed by the possibility that you may leave the political scene in Cuba before ensuring that your absence will not turn into a violent and painful transition.''
With little except the most basic goods available in pesos, in which all Cubans are paid, life for many has become a desperate search for dollars in order to buy shoes, clothes, soap and all but the most basic foods. Despite a reported crackdown earlier this year, male and female prostitution is rife.
Beautiful mulattos, often teenagers and known here as jockeys, offer to have sex with male tourists in return for a meal, sometimes only a sandwich, or for leftover toiletries. Money can be seen changing hands, however, at hotel lifts where security men demand up to $20 (pounds 13) to allow local girls in. The money, the girls say, goes to the state-employed hotel managers.
Cubans are increasingly bothered about discrimination against them in favour of tourists. When I took my "illegal" taxi driver Walfrido - at $20 a day for his beaten-up Lada he was charging one-seventh the price demanded by state taxis - to a dollar pizzeria in the swanky Marina Hemingway, he and the waiter, both mulattos, appeared equally uncomfortable. With a checkpoint at the marina entrance, few ordinary Cubans get to see such places, where a pizza costs the equivalent of several months' salary.